Display device, driving method therefor, and electronic apparatus

ABSTRACT

A display device includes a pixel array and a drive unit that drives the pixel array. The pixel array includes first and second scanning lines in rows, signal lines in columns, a matrix of pixels arranged at respective intersections of the scanning lines and the signal lines, power supply lines that supply power to each of the pixels, and ground lines. The drive unit includes a first scanner that sequentially supplies first control signals to the corresponding first scanning lines to perform line-sequential scanning on the pixels on a row-by-row basis, a second scanner that sequentially supplies second control signals to the corresponding second scanning lines in synchronization with the line-sequential scanning, and a signal selector that supplies video signals to the signal lines in synchronization with the line-sequential scanning. Each pixel includes a light-emitting element, a sampling transistor, a drive transistor, a switching transistor, and a pixel capacitor.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplication JP 2007-078218 filed in the Japanese Patent Office on Mar.26, 2007, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device for displaying animage by current-driving light-emitting elements disposed to itsrespective pixels, to a driving method for the display device and to anelectronic apparatus including the display device. More specifically,the present invention relates to a driving method for an active matrixdisplay device in which the current passing through a light-emittingelement, such as an organic electroluminescent (EL) element, iscontrolled by an insulated-gate field-effect transistor in each pixelcircuit.

2. Description of the Related Art

An example of such display device is a liquid crystal display in whichmany liquid crystal pixels are arranged in a matrix. According to imageinformation, the liquid crystal display controls the intensity of lighttransmitted through or reflected by each of the pixels, and thusdisplays an image corresponding to the image information. An organic ELdisplay, including organic EL elements as pixels, has a mechanismsimilar to that of the liquid crystal display described above. However,unlike the liquid crystal pixels of the liquid crystal display, theorganic EL elements of the organic EL display are self-luminous.Therefore, the organic EL display has advantages over the liquid crystaldisplay in that it provides better viewability, requires no backlight,and has a higher response speed. Additionally, the organic EL display isvery different from the liquid crystal display in that, unlike theliquid crystal display, which is a voltage-controlled display, theorganic EL display is a current-controlled display in which theluminance (gradation) of each light-emitting element is controllable bythe value of a current flowing therethrough.

As in the case of the liquid crystal display, there are two types ofdriving methods for the organic EL display: a simple matrix type and anactive matrix type. Although a simple-matrix display is simple instructure, it has problems in its large size and its difficultyachieving high definition display. Therefore, current efforts areprimarily directed toward the development of active-matrix displays. Inan active-matrix display, a current flowing through a light-emittingelement in each pixel circuit is controlled by an active element(typically a thin-film transistor or TFT) disposed in the pixel circuit(see, for example, Japanese Unexamined Patent Application PublicationsNos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, 2004-093682,and 2006-215213).

SUMMARY OF THE INVENTION

Pixel circuits of the related art are arranged in respectiveintersections of rows of scanning lines for supplying control signalsand columns of signal lines for supplying video signals. Each pixelcircuit includes at least a sampling transistor, a pixel capacitor, adrive transistor, and a light-emitting element. In response to a controlsignal supplied from a scanning line, the sampling transistor is broughtinto conduction and samples a video signal supplied from a signal line.The pixel capacitor holds an input voltage corresponding to a signalpotential of the sampled video signal. According to the input voltageheld by the pixel capacitor, the drive transistor supplies an outputcurrent as a drive current during a predetermined light-emitting period.Generally, the output current is dependent on carrier mobility andthreshold voltage in a channel region of the drive transistor. Inresponse to the output current supplied from the drive transistor, thelight-emitting element emits light at an intensity corresponding to thevideo signal.

The drive transistor receives, the input voltage held in the pixelcapacitor at the gate thereof, causing the output current to flowbetween the source and drain thereof, and energizes the light-emittingelement. Generally, the intensity of light emitted from thelight-emitting element is proportional to the amount of current flowingtherethrough. The amount of output current supplied from the drivetransistor is controlled by the gate voltage, that is, by the inputvoltage written to the pixel capacitor. The pixel circuit of the relatedart controls the amount of current supplied to the light-emittingelement by varying the input voltage applied to the gate of the drivetransistor according to the input video signal.

The operating characteristic of the drive transistor can be expressed byEquation 1 as follows:Ids=(½)μ(W/L)Cox(Vgs−Vth)²  Equation 1where Ids represents the drain current flowing between the source anddrain of the drive transistor, the drain current being the outputcurrent supplied to the light-emitting element in the pixel circuit; Vgsrepresents the gate voltage applied to the gate with respect to thesource with the gate voltage being the above-described input voltage inthe pixel circuit; Vth represents the threshold voltage of thetransistor; μ represents the mobility of a semiconductor thin filmserving as a channel of the transistor; W represents the channel width;L represents the channel length; and Cox represents the gatecapacitance. As can be seen from Equation 1 above, when the TFT operatesin a saturation region, if the gate voltage Vgs increases to exceed thethreshold voltage Vth, the transistor is turned on and causes the draincurrent Ids to flow. In principle, as indicated by Equation 1, if thegate voltage Vgs is constant, the drain current Ids is supplied at aconstant rate to the light-emitting element. Therefore, if video signalsof the same level are supplied to respective pixels of the screen, allthe pixels should emit light at the same intensity, thus achievingluminance uniformity over the screen.

In practice, however, there are variations in device characteristicsamong TFTs which are made of semiconductor thin films, such aspolysilicon films. In particular, the threshold voltage Vth is notconstant and varies from pixel to pixel. As can be seen from Equation 1above, even if the gate voltage Vgs is constant, variations in thresholdvalue Vth among drive transistors cause variations in drain current Idsand the luminance from pixel to pixel, thus degrading the luminanceuniformity over the screen. There has been pixel circuits developedhaving a function of canceling variations in threshold voltage amongdrive transistors. An example is disclosed in Japanese Unexamined PatentApplication Publication No. 2004-133240.

However, variations in output current to the light-emitting element arenot only caused by variations in threshold voltage Vth among drivetransistors. As can be seen from Equation 1 described above, the outputcurrent Ids varies if the mobility μ varies among drive transistors. Asa result, the uniformity of luminance over the screen is degraded. Therehas been pixel circuits developed having a function of correctingvariations in mobility among drive transistors. An example is disclosedin Japanese Unexamined Patent Application Publication No. 2006-215213.

In a pixel circuit having the mobility correcting function of relatedart, a drive current that flows through a drive transistor according toa signal potential is supplied to a pixel capacitor through negativefeedback during a predetermined correction period. Thus, the signalpotential stored in the pixel capacitor is adjusted. If the mobility ofthe drive transistor is high, the amount of negative feedback is large.In this case, the signal potential is greatly reduced, thus suppressingthe drive current. On the other hand, if the mobility of the drivetransistor is low, the amount of negative feedback to the pixelcapacitor is small. In this case, since the stored signal potential isnot greatly reduced, there is no significant reduction in drive current.Thus, depending on the level of mobility of the drive transistor in eachpixel, the signal potential is adjusted in the direction of cancelingit. Therefore, even if the mobility of the drive transistor varies frompixel to pixel, the pixels exhibit substantially the same level oflight-emitting luminance with respect to the same signal potential.

The mobility correction described above is performed during apredetermined mobility correction period. If the mobility correctionperiod varies from pixel to pixel, the amount of negative feedback alsovaries, thus performing accurate mobility correction becomes difficult.The mobility correction period is determined by on/off controlling thesampling transistor and the switching transistor according to apredetermined sequence. However, the phase of a control signal (gatepulse) for on/off controlling these transistors is not necessarilyconstant and fluctuates to some extent. This causes the mobilitycorrection period to vary from pixel to pixel, which is a problem to besolved.

With the technical disadvantage of the related art described above, itis desirable to provide a display device and a driving method for thedisplay device capable of precisely controlling the period of correctingthe mobility of a drive transistor. More specifically, it is desirableto suppress variations in mobility correction period, thereby enhancingthe uniformity of luminance over the screen of the display device. Adisplay device, according to an embodiment of the present invention,includes a pixel array and a drive unit configured to drive the pixelarray. The pixel array includes a plurality of first scanning lines andsecond scanning lines arranged in rows, a plurality of signal linesarranged in columns, a matrix of pixels arranged at respectiveintersections of the scanning lines and the signal lines, a plurality ofpower supply lines that supply power to each of the pixels, and aplurality of ground lines. The drive unit includes a first scanner thatsequentially supplies first control signals to the corresponding firstscanning lines to perform line-sequential scanning on the pixels on arow-by-row basis; a second scanner that sequentially supplies secondcontrol signals to the corresponding second scanning lines insynchronization with the line-sequential scanning; and a signal selectorthat supplies video signals to the columns of signal lines insynchronization with the line-sequential scanning. Each of the pixelsincludes a light-emitting element, a sampling transistor, a drivetransistor, a switching transistor, and a pixel capacitor. A gate of thesampling transistor is connected to one of the first scanning lines, thesource of the sampling transistor is connected to one of the signallines, and the drain of the sampling transistor is connected to the gateof the drive transistor. The drive transistor and the light-emittingelement are connected in series between one of the power supply linesand one of the ground lines to form a current path. The switchingtransistor is disposed in the current path and a gate of the switchingtransistor is connected to one of the second scanning lines. The pixelcapacitor is disposed between the source and gate of the drivetransistor. The sampling transistor is turned on in response to a firstcontrol signal supplied from the first scanning line, samples a signalpotential of a video signal supplied from the signal line, and storesthe sampled signal potential in the pixel capacitor. The switchingtransistor is turned on in response to a second control signal suppliedfrom the second scanning line and brings the current path intoconduction. The drive transistor causes a drive current to flow into thelight-emitting element through the current path placed in a state ofconduction, where the drive current depending on the signal potentialstored in the pixel capacitor. The first scanner applies a first controlsignal to the first scanning line to turn on the sampling transistor andstart sampling a signal potential. Then the first control signal appliedto the first scanning line is cancelled so as to turn off the samplingtransistor. During a video signal writing period from the time when thesampling transistor is turned on to the time when the samplingtransistor is turned off, the second scanner applies a pulsed secondcontrol signal to the second scanning line to keep the switchingtransistor on for a limited correction period, and adjusts the signalpotential stored in the pixel capacitor to correct a mobility of thedrive transistor.

After the sampling transistor is turned off and the video signal writingperiod ends, the second scanner applies a second control signal to thesecond scanning line again to keep the sampling transistor on for apredetermined light-emitting period, and brings the current path intoconduction to cause a drive current to flow into the light-emittingelement.

According to an embodiment of the present invention, during the videosignal writing period from the time when the sampling transistor isturned on to the time when the sampling transistor is turned off, ascanner included in a peripheral driving unit applies a pulsed controlsignal to a scanning line to keep the switching transistor on for alimited period of correction time. This adjusts the signal potentialstored in the pixel capacitor so as to correct the mobility of the drivetransistor. The mobility correction period is defined by the pulse widthof the control signal applied to the gate of the switching transistor.It is possible to precisely control the mobility correction period toprevent variations in mobility correction period from pixel to pixel.Thus, luminance uniformity over the screen of the display device can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an overall configuration of adisplay device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a configuration of a pixelcircuit in the display device of FIG. 1.

FIG. 3 is a circuit diagram illustrating an operation of the pixelcircuit of FIG. 2.

FIG. 4 is a timing chart illustrating a reference example of theoperation of the pixel circuit of FIG. 3.

FIG. 5 is a circuit diagram illustrating the reference example of FIG.4.

FIG. 6 is a graph illustrating the reference example of FIG. 4.

FIG. 7 is a waveform diagram illustrating the reference example of FIG.4.

FIG. 8 is a graph illustrating the reference example of FIG. 4.

FIG. 9 is a diagram illustrating the reference example of FIG. 4.

FIG. 10 is a timing chart illustrating an operation of the displaydevice according to an embodiment of the present invention.

FIG. 11 is a waveform diagram illustrating the operation of FIG. 10.

FIG. 12 is a cross-sectional view illustrating a device structure of adisplay device according to an embodiment of the present invention.

FIG. 13 is a plan view illustrating a module configuration of a displaydevice according to an embodiment of the present invention.

FIG. 14 is a perspective view illustrating a television set including adisplay device according to an embodiment of the present invention.

FIG. 15 is a perspective view illustrating a digital still cameraincluding a display device according to an embodiment of the presentinvention.

FIG. 16 is a perspective view illustrating a notebook personal computerincluding a display device according to an embodiment of the presentinvention.

FIG. 17 is a diagram illustrating a mobile terminal apparatus includinga display device according to an embodiment of the present invention.

FIG. 18 is a perspective view illustrating a video camcorder including adisplay device according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. FIG. 1 is a schematic blockdiagram illustrating an overall configuration of a display deviceaccording to an embodiment of the present invention. As illustrated, theimage display device basically includes a pixel array 1 and a drive unitincluding a scanner part and a signal part. The pixel array 1 includesscanning lines WS, scanning lines AZ1, scanning lines AZ2 and DSarranged in rows; signal lines SL arranged in columns; a matrix of pixelcircuits 2 connected to the scanning lines WS, AZ1, AZ2, DS and to thesignal lines SL; and a plurality of power supply lines for supplying afirst potential Vss1, a second potential Vss2, and a third potential VDDnecessary for operation of each of the pixel circuits 2. The signal partincludes a horizontal selector 3, which supplies video signals to thesignal lines SL. The scanner part includes a write scanner 4, a drivescanner 5, a first correcting scanner 71 and a second correcting scanner72 that supplies control signals to the scanning lines WS, scanninglines DS, scanning lines AZ1 and AZ2, respectively, so as tosequentially scan the pixel circuits 2 on a row-by-row basis.

The write scanner 4 includes a shift register that operates in responseto a externally supplied clock signal WSCK, and sequentially transfersan externally supplied start signal WSST to output control signals WS tothe respective scanning lines WS. The drive scanner 5 also includes ashift register that operates in response to a clock signal DSCKexternally supplied, and sequentially transfers an externally suppliedstart signal DSST to sequentially output control signals DS to therespective scanning lines DS.

FIG. 2 is a circuit diagram illustrating a configuration of a pixelincluded in the image display device of FIG. 1. As illustrated, thepixel circuit 2 includes a sampling transistor Tr1, a drive transistorTrd, a first switching transistor Tr2, a second switching transistorTr3, a third switching transistor Tr4, a pixel capacitor Cs, and alight-emitting element EL. In response to the control signal suppliedfrom the corresponding scanning line WS during a predetermined samplingperiod (signal writing period), the sampling transistor Tr1 is broughtinto conduction, samples a video signal supplied from the correspondingsignal line SL, and stores the signal potential of the sampled videosignal into the pixel capacitor Cs. According to the signal potential ofthe sampled video signal, the pixel capacitor Cs applies an inputvoltage Vgs to a gate G of the drive transistor Trd. The drivetransistor Trd supplies an output current Ids corresponding to the inputvoltage Vgs to the light-emitting element EL. In response to the outputcurrent Ids supplied from the drive transistor Trd during apredetermined light-emitting period, the light-emitting element EL emitslight at an intensity corresponding to the signal potential of the videosignal.

In response to a control signal supplied from the corresponding scanningline AZ1 before the sampling period is entered, the first switchingtransistor Tr2 is brought into conduction, and sets the gate G of thedrive transistor Trd to the first potential Vss1. Similarly, in responseto a control signal supplied from the corresponding scanning line AZ2before the sampling period is entered, the second switching transistorTr3 is brought into conduction, and sets a source S of the drivetransistor Trd to the second potential Vss2. In response to a controlsignal supplied from the corresponding scanning line DS before thesampling period is entered, the third switching transistor Tr4 isbrought into conduction, connecting the drive transistor Trd to thethird potential VDD, thus causing a voltage equivalent to a thresholdvoltage Vth of the drive transistor Trd to be stored in the pixelcapacitor Cs so as to correct the effect of the threshold voltage Vth.Additionally, in response to a control signal supplied again from thescanning line DS during the light-emitting period, the third switchingtransistor Tr4 is brought into conduction, connects the drive transistorTrd to the third potential VDD, and causes the output current Ids toflow through the light-emitting element EL.

As can be seen from the above description, the pixel circuit 2 includesfive transistors Tr1 to Tr4 and Trd, one pixel capacitor Cs, and onelight-emitting element EL. The transistors Tr1 to Tr3 and Trd areN-channel polysilicon TFTs, while only the transistor Tr4 is a P-channelpolysilicon TFT. However, the present invention is not limited to this,and various combinations of both N-channel and P-channel TFTs arepossible. The light-emitting element EL, for example, is a diode organicEL device having an anode and a cathode. However, the present inventionis not limited to this. The light-emitting element EL may be any kind ofgeneral device that is current-driven to emit light.

According to a feature of the present invention, during a video signalwriting period (sampling period) from when the sampling transistor Tr1is turned on to the time when the sampling transistor Tr1 is turned off,the drive scanner 5 applies a pulsed control signal to the scanning lineDS to keep the switching transistor Tr4 on during a limited correctionperiod t, and adjusts the signal potential stored in the pixel capacitorCs so as to correct a mobility μ of the drive transistor Trd.

FIG. 3 is a schematic view of the pixel circuit 2 taken out of the imagedisplay device illustrated in FIG. 2. For ease of understanding, asignal potential Vsig of the video signal sampled by the samplingtransistor Tr1, the input voltage Vgs and output current Ids of thedrive transistor Trd, and a capacitance component Coled of thelight-emitting element EL are added to FIG. 3. Hereinafter, theoperation of the pixel circuit 2 according to an embodiment of thepresent invention will be described with reference to FIG. 3.

FIG. 4 is a timing chart for the pixel circuit 2 of FIG. 3. The timingchart of FIG. 4 shows a reference example of the operation of the pixelcircuit 2 illustrated FIG. 3. To clarify the operational effect of thepresent invention, the reference example shown in FIG. 4 will bedescribed first, for purposes of comparison with the present invention.FIG. 4 shows waveforms of control signals applied to the respectivescanning lines WS, AZ1, AZ2, and DS along a time axis T. Forsimplification, the control signals are indicated by the same referencecharacters as those indicating the corresponding scanning lines. Thetransistors Tr1, Tr2, and Tr3, which are N-channel transistors, are onwhile the control signals WS, AZ1, and AZ2 are high, and off while thecontrol signals WS, AZ1, and AZ2 are low. On the other hand, thetransistor Tr4, which is a P-channel transistor, is off while thecontrol signal DS is high, and on while the control signal DS is low. Inaddition to the waveforms of the control signals WS, AZ1, AZ2, and DS,the timing chart of FIG. 4 shows changes in the potentials of the gate Gand source S of the drive transistor Trd.

In the timing chart of FIG. 4, one field (1 f) starts at time T1 andends at time T8. During the period of one field, the rows of the pixelarray are sequentially scanned once. The timing-chart of FIG. 4 showsthe waveforms of the control signals WS, AZ1, AZ2, and DS applied to onerow of pixels.

At time T0 before the field (1 f), all the control signals WS, AZ1, AZ2,and DS are at low levels. This means that the N-channel transistors Tr1,Tr2, and Tr3 are off, while only the P-channel transistor Tr4 is on.Since the drive transistor Trd is connected to the power supply VDD viathe switching transistor Tr4, which is on, the drive transistor Trdsupplies the output current Ids to the light-emitting element ELaccording to the predetermined input voltage Vgs. This causes thelight-emitting element EL to emit light at time T0. The input voltageVgs applied to the drive transistor Trd at this point can be expressedas the difference between a gate potential (G) and a source potential(S).

At time T1 when the field starts, the control signal DS goes from low tohigh. Since this causes the switching transistor Tr4 to be turned offand also causes the drive transistor Trd to be disconnected from thepower supply VDD, light emission is stopped and a non-light-emittingperiod is entered. Therefore, during the period starting at time T1, allthe transistors Tr1 to Tr4 are off.

Next, at time T2, the control signals AZ1 and AZ2 go high, which causesthe switching transistors Tr2 and Tr3 to turn on. As a result, the gateG of the drive transistor Trd is connected to the reference potentialVss1 and the source S of the drive transistor Trd is connected to thereference potential Vss2. By satisfying the conditions Vss1−Vss2>Vth andVss1−Vss2=Vgs>Vth, a preparation for a Vth correction to be performed attime T3 is made. In other words, the period from time T2 to time T3corresponds to a reset period for the drive transistor Trd.Additionally, the condition VthEL>Vss2 is satisfied, where VthELrepresents the threshold voltage of the light-emitting element EL.Therefore, a negative bias is applied to the light-emitting element EL,which is thus brought into a reverse-biased state. Entering thereverse-biased state is necessary for proper operation of the Vthcorrection and mobility correction to be performed later.

Immediately after the control signal AZ2 goes low, the control signal DSgoes low at time T3. Thus, the transistor Tr3 is turned off and thetransistor Tr4 is turned on. As a result, the drain current Ids flowsinto the pixel capacitor Cs to cause the Vth correction to start. Atthis point, the gate G of the drive transistor Trd is held at Vss1, andthe drain current Ids keeps flowing until the drive transistor Trd iscut off. After the drive transistor Trd is cut off, the source potential(S) of the drive transistor Trd becomes equal to Vss1−Vth. After thedrain current Ids is cut off, at time T4, the control signal DS goeshigh again and the switching transistor Tr4 is turned off. Then, thecontrol signal AZ1 also goes low again and the switching transistor Tr2is also turned off. As a result, the threshold voltage Vth is stored inthe pixel capacitor Cs. The period from time T3 to time T4 is a periodin which the threshold voltage Vth of the drive transistor Trd isdetected. Here, the detection period from time T3 to time T4 is referredto as a Vth correction period.

After the Vth correction is made, at time T5, the control signal WS goeshigh, the sampling transistor Tr1 is turned on, and the video signalVsig is written to the pixel capacitor Cs. The pixel capacitor Cs issufficiently smaller than the equivalent capacitance Coled of thelight-emitting element EL. Therefore, the video signal Vsig is mostlywritten to the pixel capacitor Cs. More precisely, the differencebetween the video signal Vsig and the reference potential Vss1,Vsig−Vss1, is written to the pixel capacitor Cs. Therefore, thegate-to-source voltage Vgs between the gate G and source S of the drivetransistor Trd becomes equal to (Vsig−Vss1+Vth), which is the sum of thepreviously detected and stored threshold voltage Vth and the presentlysampled difference Vsig−Vss1. If the reference potential Vss1 is set to0 V (Vss1=0 V) for ease of explanation, the gate-to-source voltage Vgsbecomes equal to Vsig+Vth as shown in the timing chart of FIG. 4. Thesampling of the video signal Vsig continues until time T7 when thecontrol signal WS goes low again. That is, the period from time T5 totime T7 corresponds to the sampling period (signal writing period).

At time T6 before time T7 when the sampling period ends, the controlsignal DS goes low and the switching transistor Tr4 is turned on. Sincethis causes the drive transistor Trd to be connected to the power supplyVDD, the process in the pixel circuit proceeds from thenon-light-emitting period to the light-emitting period. In the periodfrom time T6 to time T7 in which the sampling transistor Tr1 remains onand the switching transistor Tr4 is turned on, the mobility of the drivetransistor Trd is corrected. In other words, in the present referenceexample, the mobility correction is performed in the period from time T6to time T7 where the end of the sampling period coincides with thebeginning of the light-emitting period. At the beginning of thelight-emitting period where the mobility correction is performed, thelight-emitting element EL does not actually emit light because it isreverse-biased. In the mobility correction period from time T6 to timeT7, the drain current Ids flows through the drive transistor Trd whilethe gate G of the drive transistor Trd is fixed at the level of thevideo signal Vsig. When the condition Vss1−Vth<VthEL is satisfied, thelight-emitting element EL is reverse-biased and exhibits simplecapacitance characteristics, not diode characteristics. Thus, thecurrent Ids flowing through the drive transistor Trd is written to acapacitance C=Cs+Coled, which is the combination of the pixel capacitorCs and the equivalent capacitance Coled of the light-emitting elementEL. This causes the source potential (S) of the drive transistor Trd toincrease by ΔV, as shown in the timing chart of FIG. 4. The increase ΔVis eventually subtracted from the gate-to-source voltage Vgs stored inthe pixel capacitor Cs, which means that negative feedback is applied.Thus, by supplying the output current Ids of the drive transistor Trd tothe input voltage Vgs of the drive transistor Trd through negativefeedback, the mobility μ can be corrected. The amount of negativefeedback ΔV can be optimized by adjusting the duration t of the mobilitycorrection period from time T6 to time T7.

At time T7, the control signal WS goes low and the sampling transistorTr1 is turned off. This causes the gate G of the drive transistor Trd tobe disconnected from the signal line SL. Since the application of thevideo signal Vsig is cancelled, the gate potential (G) of the drivetransistor Trd increases together with the source potential (S) thereof.During the period in which the gate potential (G) and the sourcepotential (S) increase, the gate-to-source voltage Vgs stored in thepixel capacitor Cs maintains the value of (Vsig−ΔV+Vth). As the sourcepotential (S) increases, the reverse-biased state of the light-emittingelement EL is cancelled. Therefore, when the output current Ids flowsinto the light-emitting element EL, the light-emitting element ELactually starts emitting light. By substituting Vsig−ΔV+Vth into Vgs ofEquation 1, the relationship between the drain current Ids and the gatevoltage Vgs can be given by Equation 2 as follows:Ids=kμ(Vgs−Vth)² =kμ(Vsig−ΔV)²  Equation 2where k=(½)(W/L)Cox. Equation 2 indicates that the term Vth is canceled,and the output current Ids supplied to the light-emitting element EL isnot dependent on the threshold voltage Vth of the drive transistor Trd.Basically, the drain current Ids is determined by the signal voltageVsig of the video signal. In other words, the light-emitting element ELemits light at an intensity depending on the video signal Vsig, which iscorrected with the amount of negative feedback ΔV. The amount ofcorrection ΔV acts to cancel the effect of the mobility μ located in thecoefficient part of Equation 2. Therefore, the drain current Ids isdependent only on the video signal Vsig.

Last, at time T8, the control signal DS goes high and the switchingtransistor Tr4 is turned off. Upon completion of light emission, thepresent field ends. In the subsequent field, the Vth correction process,the mobility correction process, and the light-emitting process arerepeated.

FIG. 5 is a circuit diagram illustrating a state of the pixel circuit 2in the mobility correction period from time T6 to time T7. Asillustrated, the sampling transistor Tr1 and the third switchingtransistor Tr4 are on in the mobility correction period from time T6 totime T7, while the remaining switching transistors Tr2 and Tr3 are off.In this state, the source potential (S) of the drive transistor Trd canbe expressed as Vss1−Vth. The source potential (S) also serves as theanode potential of the light-emitting element EL. As described above,when the condition Vss1−Vth<VthEL is satisfied, the light-emittingelement EL is reverse-biased and exhibits simple capacitancecharacteristics, not diode characteristics. Thus, the current Idsflowing through the drive transistor Trd flows into the capacitanceC=Cs+Coled, which is the combination of the pixel capacitor Cs and theequivalent capacitance Coled of the light-emitting element EL. In otherwords, part of the drain current Ids is supplied to the pixel capacitorCs through negative feedback, thus performing mobility correction.

FIG. 6 shows Equation 2 in graphical form. The vertical axis of thegraph represents Ids and the horizontal axis of the graph representsVsig. Equation 2 is also presented under the graph. In the graph of FIG.6, characteristic curves for Pixel 1 and Pixel 2 are plotted forcomparison purposes. The mobility μ of a drive transistor included inPixel 1 is relatively high, while the mobility μ of a drive transistorincluded in Pixel 2 is relatively low. Thus, when the drive transistorsare polysilicon TFTs or the like, the mobility μ inevitably variesbetween the pixels. For example, if the signal potentials of the videosignals Vsig having the same level are written to Pixels 1 and 2 and nomobility correction is made, there will be a considerable differencebetween an output current i′ flowing through Pixel 1 having a highermobility μ and an output current Ids2′ flowing through Pixel 2 which hasa lower mobility μ. Since variations in mobility μ cause a considerabledifference between the output currents Ids, streaky unevenness may occurand luminance uniformity over the screen will be degraded.

Therefore, in the present reference example, variations in mobility arecancelled by supplying the output current to the input voltage throughnegative feedback. As can be seen from Equation 1, the higher themobility, the larger the drain current Ids. This means that the higherthe mobility, the larger the amount of negative feedback ΔV. As shown inthe graph of FIG. 6, the amount of negative feedback ΔV1 for Pixel 1having a higher mobility μ is larger than the amount of negativefeedback ΔV2 for Pixel 2 having a lower mobility μ. That is, a lageramount of negative feedback is applied to a pixel having a highermobility μ, and variations in mobility μ can be suppressed. As shown inFIG. 6, if the mobility is corrected by ΔV1 for Pixel 1 having a highermobility μ, the output current is significantly reduced from Ids1′ toIds1. On the other hand, since the amount of correction ΔV2 for Pixel 2having a lower mobility μ is smaller, the output current is reduced fromIds2′ to Ids2, which is not significant. Consequently, Ids1 and Ids2become substantially equal, and variations in mobility are cancelled.Since the cancellation of mobility variations is performed over theentire range of Vsig from a black level to a white level, the luminanceuniformity over the screen is made extremely high. In summary, if thereare Pixels 1 and 2 with different mobilities, the amount of correctionΔV1 for Pixel 1 having a higher mobility μ is larger than the amount ofcorrection ΔV2 for Pixel 2 having a lower mobility μ. In other words,the higher the mobility, the larger the amount of correction ΔV and thusa greater reduction in the output current Ids. As a result, the valuesof currents flowing through pixels having different mobilities are madeuniform, and variations in mobility can be corrected.

For reference purposes, a numerical analysis of the above mobilitycorrection will be described. The analysis is performed while thetransistors Tr1 and Tr4 are on, as illustrated in FIG. 5. Here, thesource potential of the drive transistor Trd is used as a variable V.The drain current Ids flowing through the drive transistor Trd isexpressed by Equation 3 as follows:I _(ds) =kμ(V _(gs) −V _(th))² =kμ(V _(sig) −V−V _(th))²  Equation 3where V represents the source potential (S) of the drive transistor Trd.

On the basis of the relationship between the drain current Ids and thecapacitance C (=Cs+Coled), Ids=dQ/dt=CdV/dt is satisfied, as indicatedby Equation 4 below:

$\begin{matrix}{{{{From}\mspace{14mu} I_{ds}} = {\frac{\mathbb{d}Q}{\mathbb{d}t} = {C\frac{\mathbb{d}V}{\mathbb{d}t}}}},{{\int{\frac{1}{C}{\mathbb{d}t}}} = {\left. {\int{\frac{1}{I_{ds}}{\mathbb{d}V}}}\Leftrightarrow{\int_{0}^{t}{\frac{1}{C}\ {\mathbb{d}t}}} \right. = {\left. {\int_{- {Vth}}^{V}{\frac{1}{k\;{\mu\left( {V_{sig} - V_{th} - V} \right)}^{2}}\ {\mathbb{d}V}}}\Leftrightarrow{\frac{k\;\mu}{C}t} \right. = {\left\lbrack \frac{1}{V_{sig} - V_{th} - V} \right\rbrack_{- {Vth}}^{V} = {\left. {\frac{1}{V_{sig} - V_{th} - V} - \frac{1}{V_{sig}}}\Leftrightarrow{V_{sig} - V_{th} - V} \right. = {\frac{1}{\frac{1}{V_{sig}} + {\frac{k\;\mu}{C}t}} = \frac{V_{sig}}{1 + {V_{sig}\frac{k\;\mu}{C}t}}}}}}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Then, Equation 3 is substituted into Equation 4 and both sides of theresulting equation are integrated, where −Vth is the initial value ofthe source voltage V and t is the mobility variation correction period(from time T6 to time T7) for correcting variations in mobility. Solvingthis differential equation gives Equation 5, which expresses the pixelcurrent with respect to the mobility correction period t as follows:

$\begin{matrix}{I_{ds} = {k\;{\mu\left( \frac{V_{sig}}{1 + {V_{sig}\frac{k\;\mu}{C}t}} \right)}^{2}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

As described above, the output current that flows through thelight-emitting element in each pixel is expressed by Equation 5 above.In Equation 5, the mobility correction period μ is set to severalmicroseconds (μm). As described above, the mobility correction period tis determined by the interval between turn-on time (falling time) of theswitching transistor Tr4 and turn-off time (falling time) of thesampling transistor Tr1. FIG. 7 shows, along the time axis, a fallingwaveform of the control signal DS applied to the gate of the switchingtransistor Tr4 and a falling waveform of the control signal WS appliedto the gate of the sampling transistor Tr1. The scanning lines throughwhich the control signals DS and WS are transmitted are pulse wires madeof a material having a relatively high resistance, such as metallicmolybdenum. Since the overlap parasitic capacitance between wires onadjacent layers is large, the time constant of the pulse wires is large,which makes the falling waveforms of the control signals DS and WS lesssteep. That is, the control signals DS and WS do not fallinstantaneously, but fall rather gradually from the power supplypotential Vcc to the ground potential Vss, due to the effect of the timeconstant determined by wiring capacitance and wiring resistance. Thefalling waveforms are applied to the gates of the switching transistorTr4 and sampling transistor Tr1.

On the other hand, the signal potential Vsig is supplied to the sourceof the sampling transistor Tr1. Therefore, the sampling transistor Tr1is turned off when the gate potential falls below Vsig+Vtn, where Vtnrepresents the threshold voltage of the N-channel sampling transistorTr1. Similarly, the source of the switching transistor Tr4 is connectedto the power supply potential VDD of the pixel. Therefore, the switchingtransistor Tr4 is turned on when the gate potential of the switchingtransistor Tr4 drops to VDD−|Vtp|, where Vtp represents the thresholdvoltage of the P-channel switching transistor Tr4.

The falling waveform of the control signal DS varies. In the lower partof FIG. 7, (1) indicates a normal phase, while (2) indicates the worstcase in which the slope of the falling waveform becomes steeper. Suchvariations in the falling waveform of the control signal DS causevariations in the turn-on time of the switching transistor Tr4. Thefalling waveform of the control signal WS also varies. In the upper partof FIG. 7, (1) indicates a normal phase, while (2) indicates the worstcase in which the slope of the falling waveform becomes less steep. Suchvariations in the falling waveform of the control signal WS causevariations in the turn-off time of the sampling transistor Tr1. If theturn-on time of the switching transistor Tr4 and the turn-off time ofthe sampling transistor Tr1 are shifted in the opposite directions as inthe worst cases described above, the mobility correction period tdefined by the interval between these time points is considerablyshifted from that in the case of the normal phases. As a result, thisappears as variations in the intensity of emitted light.

FIG. 8 is a graph showing the relationship between the mobilitycorrection period and the drive current (pixel current) flowing througha pixel. In the graph of FIG. 8, the horizontal axis represents themobility correction period and the vertical axis represents the pixelcurrent. As can be seen from the graph, if the mobility correctionperiod varies, the pixel current also varies from pixel to pixel, thusdegrading the luminance uniformity over the screen. As described above,variations in mobility correction period are primarily caused byvariations in the transient response of the control signals applied tothe gates of the sampling transistor Tr1 and switching transistor Tr4.

FIG. 9 is a diagram for explaining the cause of variations in thetransient response of the control signals described above. Asillustrated in FIG. 9, the display device is composed of a singleinsulating substrate, which is a flat panel 0 on which the write scanner4, the drive scanner 5, and the horizontal selector 3 are formed aroundthe pixel array 1 in an integrated manner. Like the pixel array 1 in thecenter, these peripheral drive units are formed of TFTs in an integratedmanner. Generally, a TFT includes a polysilicon layer as a device area.The polysilicon layer is produced, for example, by forming an amorphoussilicon thin film on an insulating substrate, and applying laser lightto the amorphous silicon thin film to crystallize and transform it intothe polysilicon layer. In the process of application of laser light, forexample, a linear laser beam (excimer laser annealing or ELA) issequentially applied to the panel 0 in the downward direction thereof inan overlaying manner, thus transforming the amorphous silicon film intothe polysilicon layer. If there are local variations in the laser outputduring the process of laser light application, the crystallinity of thepolysilicon layer varies depending on the position in the up-and-downdirection of the panel 0. This results in variations in characteristicsamong TFTs. Typically, such variations in characteristics appear in thehorizontal direction of the panel 0 along the path of laser light. Inthe example of FIG. 9, a correction period in some lines of the panel 0is different from that in the other lines, because the characteristicsof the corresponding transistors, which are some of transistors servingas the output stages of the scanners, are different from those of theothers. As shown in FIG. 8, since variations in correction period leadto variations in pixel current, unevenness in luminance occurs along thelines. If the correction period is shorter than the average, the amountof negative feedback for a signal potential is small, which causes astreak brighter than its surroundings to appear. On the other hand, ifthe correction period is longer than the average, the amount of negativefeedback for a signal potential is large, which lowers the signalpotential and causes a streak darker than its surroundings to appear.

Referring to FIG. 9, the output stages of the write scanner 4 are in aone-to-one correspondence with, and are aligned with the output stagesof the drive scanner 5. If the corresponding output stages between thewrite scanner 4 and the drive scanner 5 are aligned with each other onthe same line, there will be no significant phase difference between thecontrol signals output from both scanners. However, if the correspondingoutput stages of the write scanner 4 and drive scanner 5 go out ofalignment even to a slight degree, the application conditions of thelaser beam (ELA) are shifted accordingly. This causes a phase differenceand variations in transient response between the outputs from the writescanner 4 and drive scanner 5. As a result, the mobility correctionperiod determined by the time interval between the control signal fromthe write scanner 4 and that from the drive scanner 5 also varies.

FIG. 10 is a timing chart for explaining the operation of the displaydevice illustrated in FIG. 1 to FIG. 3, according to an embodiment ofthe present invention. For ease of understanding, FIG. 10 uses referencecharacters identical to those used in FIG. 4. In the timing chart ofFIG. 10, the mobility correction period is determined by only thecontrol signal DS output from the drive scanner 5 unlike in the case ofthe reference example illustrated in FIG. 4. This makes it possible tosuppress variations in mobility correction period, which is describedabove in the reference example. Hereinafter, the operation of thedisplay device according to an embodiment of the present invention willbe described in detail with reference to FIG. 10.

At time T1 when the field starts, the control signal DS goes from low tohigh. Since this causes the switching transistor Tr4 to be turned offand also causes the drive transistor Trd to be disconnected from thepower supply VDD, light emission is stopped and a non-light-emittingperiod is entered. Therefore, during the period starting at time T1, allthe transistors Tr1 to Tr4 are off.

Next, at time T2, the control signals AZ1 and AZ2 go high, which causesthe switching transistors Tr2 and Tr3 to be turned on. As a result, thegate G of the drive transistor Trd is connected to the referencepotential Vss1 and the source S of the drive transistor Trd is connectedto the reference potential Vss2. By satisfying the conditions,Vss1−Vss2>Vth and Vss1−Vss2=Vgs>Vth, a preparation for a Vth correctionto be performed at time T3 is made. In other words, the period from timeT2 to time T3 corresponds to a reset period for the drive transistorTrd. Additionally, the condition VthEL>Vss2 is satisfied, where VthELrepresents the threshold voltage of the light-emitting element EL.Therefore, a negative bias is applied to the light-emitting element EL,which is then brought into a reverse-biased state. Entering thereverse-biased state is necessary for proper operation of the Vthcorrection and mobility correction to be performed later.

Immediately after the control signal AZ2 goes low, the control signal DSgoes low at time T3. Thus, the transistor Tr3 is turned off and thetransistor Tr4 is turned on. As a result, the drain current Ids flowsinto the pixel capacitor Cs to cause the Vth correction to start. Atthis point, the gate G of the drive transistor Trd is held at Vss1, andthe drain current Ids keeps flowing until the drive transistor Trd iscut off. After the drive transistor Trd is cut off, the source potential(S) of the drive transistor Trd is made equal to Vss1−Vth. After thedrain current Ids is cut off, at time T4, the control signal DS goeshigh again and the switching transistor Tr4 is turned off. Then, thecontrol signal AZ1 goes low again and the switching transistor Tr2 isalso turned off. As a result, the threshold voltage Vth is stored in thepixel capacitor Cs. The period from time T3 to time T4 is a period inwhich the threshold voltage Vth of the drive transistor Trd is detected.Here, the detection period from time T3 to time T4 is referred to as theVth correction period.

After the Vth correction is made, at time T5, the control signal WS goeshigh, the sampling transistor Tr1 is turned on, and the video signalVsig is written to the pixel capacitor Cs. The pixel capacitor Cs issufficiently smaller than the equivalent capacitance Coled of thelight-emitting element EL. Therefore, the video signal Vsig is mostlywritten to the pixel capacitor Cs. More precisely, the differencebetween the video signal Vsig and the reference potential Vss1,Vsig−Vss1, is written to the pixel capacitor Cs. Therefore, thegate-to-source voltage Vgs between the gate G and source S of the drivetransistor Trd becomes equal to (Vsig−Vss1+Vth), which is the sum of thepreviously detected and stored threshold voltage Vth and the presentlysampled difference Vsig−Vss1. If the reference potential Vss1 is set to0 V (Vss1=0 V) for ease of explanation, the gate-to-source voltage Vgsbecomes equal to Vsig+Vth as shown in the timing chart of FIG. 10. Thesampling of the video signal Vsig continues until time T8 when thecontrol signal WS goes low again. That is, the period from time T5 totime T8 corresponds to the sampling period.

Before time T8 at which the sampling period (video signal writingperiod) ends, the pulsed control signal DS is applied to the scanningline DS. The pulsed control signal DS, which falls at time T6 and risesat time T7, is a negative pulse having a relatively short pulse width.In the period from time T6 to time T7, the switching transistor Tr4 isturned on and the mobility correction period is defined. The mobilitycorrection period from time T6 to time T7 is determined only by thepulse width of the control signal DS, and does not significantly varyfrom pixel to pixel. The mobility correction period from time T6 to timeT7 falls within the video signal writing period from time T5 to time T8.

As described above, in the mobility correction period from time T6 totime T7, the switching transistor Tr4 is turned on, which causes thedrive transistor Trd to be connected to the power supply VDD. At thispoint, since the sampling transistor Tr1 is on, the drain current Idsflows through the drive transistor Trd while the gate G of the drivetransistor Trd is fixed at the level of the video signal Vsig. When thecondition Vss1−Vth<VthEL is satisfied, the light-emitting element EL isreverse-biased and exhibits simple capacitance characteristics, notdiode characteristics. Thus, the drain current Ids flowing through thedrive transistor Trd is written to the capacitance C=Cs+Coled, which isthe combination of the pixel capacitor Cs and the equivalent capacitanceColed of the light-emitting element EL. This causes the source potential(S) of the drive transistor Trd to increase by ΔV, as shown in thetiming chart of FIG. 10. The increase ΔV is eventually subtracted fromthe gate-to-source voltage Vgs stored in the pixel capacitor Cs, whichmeans that negative feedback is applied. Thus, by supplying the outputcurrent Ids of the drive transistor Trd to the input voltage Vgs of thedrive transistor Trd through negative feedback, the mobility μ can becorrected. By precisely controlling the duration of the mobilitycorrection period from time T6 to time T7, variations in the amount ofnegative feedback ΔV among pixels can be suppressed.

At time T8, the control signal WS goes low and the sampling transistorTr1 is turned off. This causes the gate G of the drive transistor Trd tobe disconnected from the signal line SL. Then, at time T9, the controlsignal DS goes low again and the drive transistor Trd is connected tothe power supply VDD. This causes a current to flow through thelight-emitting element EL. At the same time, the source potential (S) ofthe drive transistor Trd increases, while the gate potential (G) of thedrive transistor Trd also increases in synchronization therewith. Duringthe period in which the gate potential (G) and the source potential (S)increase, the gate-to-source voltage Vgs stored in the pixel capacitorCs maintains the value of (Vsig−ΔV+Vth). As the source potential (S)increases, the reverse-biased state of the light-emitting element EL iscancelled. Therefore, when the output current Ids flows into thelight-emitting element EL, the light-emitting element EL actually startsemitting light.

FIG. 11 schematically shows changes in the waveforms of the controlsignals WS and DS observed during the period from time T6 to time T9 inthe timing chart of FIG. 10. For ease of understanding, FIG. 11 usesreference characters identical to those used in the waveform diagram ofFIG. 7.

The control signal WS is applied to the gate of the sampling transistorTr1. The control signal WS falls from Vcc to Vss at time T8. The fallingwaveform of the control signal WS varies among lines. In the upper partof FIG. 11, (1) indicates a normal state, while (2) indicates the worststate in which the slope of the falling waveform becomes less steep. Asdescribed above, the signal potential Vsig is supplied to the source ofthe sampling transistor Tr1. Therefore, the sampling transistor Tr1 isturned off when the gate potential falls below Vsig+Vtn. If the slope ofthe falling waveform of the control signal WS is less steep, fallingtime T8 will vary between the normal phase (1) and the worst phase (2).

On the other hand, the control signal DS is applied to the gate of theswitching transistor Tr4. During the period from time T6 to time T7, thecontrol signal DS is a negative pulse. At time T9, the control signal DSbecomes a negative pulse again and is applied to the scanning line DS.In the lower part of FIG. 11, (1) indicates a normal phase of thewaveform of the control signal DS, while (2) indicates the worst phasein which the slope of the waveform of the control signal DS becomessteeper, which is opposite to the case of the control signal WS.

The source of the switching transistor Tr4 is connected to the powersupply potential VDD of the pixel. Therefore, the switching transistorTr4 is turned on when the gate potential of the switching transistor Tr4drops to VDD−|Vtp|. Here, the time when the negative pulse of thecontrol signal DS crosses the level of VDD−|Vtp| varies between thenormal phase (1) and the worst phase (2). As shown in FIG. 11, fallingtime T6 and rising time T7 each vary by about Δt between the normalphase (1) and the worst phase (2). However, the direction in which theworst phase (2) is shifted from the normal phase (1) at time T6 is thesame as that at time T7. Therefore, although there are variations inboth T6 and T7 between the normal phase (1) and the worst phase (2),there is almost no variation in mobility correction period t between thenormal phase (1) and the worst phase (2). Thus, in the presentinvention, the mobility correction period is determined by only thenegative pulse of the control signal DS.

As shown in FIG. 11, during the period in which the control signal WS isat a high level and the sampling transistor Tr1 is on, the controlsignal DS is lowered and the switching transistor Tr4 is turned on.Then, during the period in which the sampling transistor Tr1 is kept on,the control signal DS is raised and the switching transistor Tr4 isturned off. After the control signal WS falls and the samplingtransistor Tr1 is turned off, the control signal DS is lowered again andthe switching transistor Tr4 is turned on, which causes thelight-emitting element EL to emit light. That is, in the presentinvention, the mobility correction is controlled only by the negativepulse of the control signal DS. Therefore, no problem arises even if theoutput characteristics vary between the corresponding output stages ofthe drive scanner 5 and the write scanner 4. The mobility correctionperiod is determined only by the pulse of the control signal DS. Sincevariations in the rising point and falling point of the pulse occur inthe same direction, it is possible to suppress variations in mobilitycorrection period. In the present invention, the mobility correctionperiod is determined only by the pulse of the control signal DS. Even ifthe transmission period during which the pulse of the control signal DSis transmitted varies, there is no operational problem as far as thetransmission period falls within the period during which the samplingtransistor Tr1 is on. Even if the transient response or phase of thecontrol signal DS varies, there is substantially no change in timedifference between the time when the switching transistor Tr4 is turnedon and the time when the switching transistor Tr4 is turned off, thuspresenting no significant variation in mobility correction period. Atthe same time, variations in the phase of the control signal WS do notaffect the mobility correction process. Therefore, even ifcharacteristics vary among transistors included in the write scanner ordrive scanner, it is possible to precisely control the mobilitycorrection period. Thus, image quality problems, such as streakyunevenness and the like, can be suppressed and images with a high degreeof luminance uniformity can be achieved.

FIG. 12 is a cross-sectional view illustrating a thin-film structure ofa display device according to an embodiment of the present invention.FIG. 12 schematically illustrates a cross section of a pixel formed onan insulating substrate. As illustrated, the pixel includes a transistorunit including a plurality of TFTs (only one TFT is shown in FIG. 12), acapacitor unit such as a hold capacitor, and a light-emitting unit suchas an organic EL element. The transistor unit and the capacitor unit areformed by a TFT process on the substrate, the light-emitting unit isformed thereon, and a transparent counter substrate is bonded theretowith an adhesive placed between the light-emitting unit and the countersubstrate, thus producing a flat panel.

The display device according to an embodiment of the present inventionmay be a flat display module illustrated in FIG. 13. For example, thedisplay module includes an insulating substrate on which a pixel arrayis disposed. The pixel array includes a matrix of pixels, each pixelhaving an organic EL element, a TFT, a thin-film capacitor, and thelike. The display module is produced by attaching a transparent countersubstrate, such as a glass substrate, to an adhesive placed around theperimeter of the pixel array (pixel matrix). If necessary, thetransparent counter substrate may be provided with a color filter, aprotective film, a light-shielding film, and the like. At the same time,the display module may be provided with a connector, such as a flexibleprinted circuit (FPC), for transmission of signals or the like betweenthe pixel array and external devices.

The display device according to the above-described embodiments of thepresent invention is a flat panel display device that can be used as adisplay for various types of electronic apparatuses (for example,digital cameras, notebook personal computers, mobile phones, and videocamcorders) capable of displaying externally input or internallygenerated drive signals as an image or video. Hereinafter, examples ofsuch electronic apparatuses will be described.

FIG. 14 illustrates a television to which the present invention isapplied. The television includes an image display screen 11 composed ofa front panel 12, a glass filter 13, and the like. The television ofFIG. 14 is realized by using a display device according to an embodimentof the present invention as the image display screen 11.

FIG. 15 illustrates a digital camera to which the present invention isapplied. The front and rear surfaces of the digital camera are presentedin the upper and lower parts, respectively, of FIG. 15. The digitalcamera includes an image pickup lens, a light-emitting unit 15 servingas a flash, a display unit 16, a control switch, a menu switch, and ashutter 19. The digital camera of FIG. 15 is realized by using a displaydevice according to an embodiment of the present invention as thedisplay unit 16.

FIG. 16 illustrates a notebook personal computer to which the presentinvention is applied. A main body 20 of the notebook personal computerincludes a keyboard for entering text and the like. A cover for the mainbody 20 includes a display unit 22 for displaying images. The notebookpersonal computer of FIG. 16 is realized by using a display deviceaccording to an embodiment of the present invention as the display unit22.

FIG. 17 illustrates a mobile terminal apparatus to which the presentinvention is applied. An open state and a folded state of the mobileterminal apparatus are presented in the left and right parts,respectively, of FIG. 17. The mobile terminal apparatus includes anupper housing 23, a lower housing 24, a joint 25 (hinge), a display 26,a sub-display 27, a picture light 28, and a camera 29. The mobileterminal apparatus of FIG. 17 is realized by using a display deviceaccording to an embodiment of the present invention as the display 26and/or the sub-display 27.

FIG. 18 illustrates a video camcorder to which the present invention isapplied. The video camcorder includes a main body 30, a lens 34 providedon the front side of the main body 30 and used for shooting a subject, astart/stop switch 35 for starting or stopping the shooting operation,and a monitor 36. The video camcorder of FIG. 18 is realized by using adisplay device according to an embodiment of the present invention asthe monitor 36.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A display device comprising: a pixel array, and adrive unit configured to drive the pixel array; wherein the pixel arrayincludes a plurality of first scanning lines and second scanning linesarranged in rows, a plurality of signal lines arranged in columns, amatrix of pixels arranged at respective intersections of the scanninglines and the signal lines, a plurality of power supply lines thatsupply power to each of the pixels, and a plurality of ground lines;wherein the drive unit includes a first scanner that sequentiallysupplies first control signals to the corresponding first scanning linesso as to perform line-sequential scanning on the pixels on a row-by-rowbasis, a second scanner that sequentially supplies second controlsignals to the corresponding second scanning lines in synchronizationwith the line-sequential scanning, and a signal selector that suppliesvideo signals to the columns of signal lines in synchronization with theline-sequential scanning; wherein each of the pixels includes alight-emitting element, a sampling transistor, a drive transistor, aswitching transistor, and a pixel capacitor; wherein each of the pixelsis configured such that: a gate of the sampling transistor is connectedto one of the first scanning lines, a source of the sampling transistoris connected to one of the signal lines, and a drain of the samplingtransistor is connected to a gate of the drive transistor, the drivetransistor and the light-emitting element are connected in seriesbetween one of the power supply lines and one of the ground lines toform a current path, the switching transistor is disposed in the currentpath and a gate of the switching transistor is connected to one of thesecond scanning lines, the pixel capacitor is disposed between thesource and gate of the drive transistor, the sampling transistor isturned on in response to a first control signal supplied from the firstscanning line, samples a signal potential of a video signal suppliedfrom the signal line, and stores the sampled signal potential in thepixel capacitor, the switching transistor is turned on in response to asecond control signal supplied from the second scanning line and bringsthe current path into conduction, and the drive transistor causes, whenin a state of conduction, a drive current to flow into thelight-emitting element through the current path, the drive currentdepending on the signal potential stored in the pixel capacitor; andwherein the drive unit is configured to drive each pixel such that: thefirst scanner applies the first control signal to the first scanningline, thereby turning on the sampling transistor and sampling a signalpotential, and then cancels the first control signal applied to thefirst scanning line so as to turn off the sampling transistor, theperiod when the sampling transistor is on defining a video signalwriting period, prior to the video signal writing period, during athreshold correction period in which a threshold correction function isperformed, the second scanner applies a first instance of the secondcontrol signal to the second scanning line, turning on the switchingtransistor, and then cancels the first instance of the second controlsignal prior to the beginning of the video signal writing period, andduring a video signal writing period, the second scanner applies asecond instance of the second control signal to the second scanningline, thereby turning the switching transistor on for a limitedcorrection period and adjusting the signal potential stored in the pixelcapacitor, and then cancels the second instance of the second controlsignal prior to the end of the video signal writing period.
 2. Thedisplay device according to claim 1, wherein after the samplingtransistor is turned off and the video signal writing period ends, andbefore the beginning of a next frame, the second scanner applies a thirdinstance of the control signal to the second scanning line, therebyturning the switching transistor on for a predetermined light-emittingperiod, bringing the current path into conduction, and causing a drivecurrent to flow into the light-emitting element; and the third instanceof the control signal is canceled at the end of the light emittingperiod.
 3. A driving method for a display device including a pixelarray, and a drive unit configured to drive the pixel array; wherein thepixel array includes a plurality of first scanning lines and secondscanning lines arranged in rows, a plurality of signal lines arranged incolumns, a matrix of pixels arranged at respective intersections of thescanning lines and the signal lines, a plurality of power supply linesthat supply power to each of the pixels, and a plurality of groundlines; wherein the drive unit includes a first scanner that sequentiallysupplies first control signals to the corresponding first scanning linesso as to perform line-sequential scanning on the pixels on a row-by-rowbasis, a second scanner that sequentially supplies second controlsignals to the corresponding second scanning lines in synchronizationwith the line-sequential scanning, and a signal selector that suppliesvideo signals to the columns of signal lines in synchronization with theline-sequential scanning; wherein each of the pixels includes alight-emitting element, a sampling transistor, a drive transistor, aswitching transistor, and a pixel capacitor; and wherein each of thepixels is configured such that: a gate of the sampling transistor isconnected to one of the first scanning lines, a source of the samplingtransistor is connected to one of the signal lines, and a drain of thesampling transistor is connected to a gate of the drive transistor, thedrive transistor and the light-emitting element are connected in seriesbetween one of the power supply lines and one of the ground lines toform a current path, the switching transistor is disposed in the currentpath and a gate of the switching transistor is connected to one of thesecond scanning lines, the pixel capacitor is disposed between thesource and gate of the drive transistor, the sampling transistor isturned on in response to a first control signal supplied from the firstscanning line, samples a signal potential of a video signal suppliedfrom the signal line, and stores the sampled signal potential in thepixel capacitor, the switching transistor is turned on in response to asecond control signal supplied from the second scanning line therebybringing the current path into conduction, and the drive transistorcauses, when in a state of conduction, a drive current to flow into thelight-emitting element through the current path, the drive currentdepending on the signal potential stored in the pixel capacitor; thedriving method comprising performing, for each pixel, the sequentialsteps of: applying a first instance of the second control signal to thesecond scanning line and thereby turning the switching transistor on andcorrecting a threshold value of the drive transistor, canceling thefirst instance of the second control signal, thereby turning off theswitching transistor, applying the first control signal to the firstscanning line, thereby turning on the sampling transistor and sampling asignal potential, applying a second instance of the second controlsignal to the second scanning line, thereby turning the switchingtransistor on for a limited correction period, and adjusting the signalpotential stored in the pixel capacitor, canceling the second instanceof the second control signal, thereby turning off the switchingtransistor, and canceling the first control signal applied to the firstscanning line, thereby turning off the sampling transistor.
 4. Anelectronic apparatus comprising the display device according to claim 1.5. The driving method for a display device according to claim 3, furthercomprising the steps of: applying a third instance of the second controlsignal to the second scanning line after the first control signal iscanceled and the sampling transistor is turned off and before thebeginning of a next frame, thereby turning the switching transistor onfor a predetermined light-emitting period, bringing the current pathinto conduction, and causing a drive current to flow into thelight-emitting element; canceling the third instance of the secondcontrol signal at the end of the predetermined light-emitting period. 6.The electronic apparatus according to claim 4, wherein after thesampling transistor is turned off and the video signal writing periodends, and before the beginning of a next frame, the second scannerapplies a third instance of the second control signal to the secondscanning line, thereby turning the switching transistor on for apredetermined light-emitting period, bringing the current path intoconduction, and causing a drive current to flow into the light-emittingelement; and the third instance of the control signal is canceled at theend of the light emitting period.
 7. The display device according toclaim 1, wherein the second instance of the second control signal isapplied after the beginning of the video signal writing period and iscanceled before the end of the video signal writing period such that aduration of the limited correction period is not affected by a transientresponse characteristic of the first scanning line, and variation in theduration of the limited correction period across pixels is suppressed.8. The driving method for a display device according to claim 3, whereinthe second instance of the second control signal is applied after thebeginning of the video signal writing period and is canceled before theend of the video signal writing period such that a duration of thelimited correction period is not affected by a transient responsecharacteristic of the first scanning line, and variation in the durationof the limited correction period across pixels is suppressed.
 9. Theelectronic apparatus according to claim 4, wherein the second instanceof the second control signal is applied after the beginning of the videosignal writing period and is canceled before the end of the video signalwriting period such that a duration of the limited correction period isnot affected by a transient response characteristic of the firstscanning line, and variation in the duration of the limited correctionperiod across pixels is suppressed.